Assistant Commissioner Mark Rowley said there are 118 ‘Syria-related’ live operations currently under way in the UK capital, including monitoring jihadists who have returned from Syria as well as those who have been radicalised at home.

The revelation comes as the Mail on Sundayreveals that suspected Islamic State links account for more than half of UK terror arrests for the first time, with more than 183 people throughout Britain being arrested on suspicion Syria-linked offences over the past year.

Mr Rowley released the figures during a private meeting with Scotland Yard chiefs that took place in August. The details of the meeting, however, have only just been revealed.

“The number of live operations [has] increased to 394 for June and SO15 [counter-terror command] are managing 11 of the 15 national priority operations. Currently 118 operations are Syria-related.” He said.

Authorities believe over 700 Britons have now travelled to Syria and Iraq to join Islamic State and other Islamist groups. Although some have died in battle, security services believe around 450 have returned to Britain.

Those who have been radicalised and battle-hardened by their experiences may now be using their newly-acquired skills to plot a terrorist attack in Britain.

Overall, there are now 600 live anti-terror operations across Britain, police have said, with around 300 linked to Islamic State.

A spokesman for Scotland Yard said: “The scale of our effort is illustrated by the large number of ongoing investigative operations, including those relating to Syria.

“The police are currently investigating hundreds of active cases, involving hundreds more individuals. Many of these investigations will be jointly run with MI5, targeting the most dangerous people and plots.”